Would have expected that this brightness would be greater in area, but it appears mostly localised to the edges of blocks. Perhaps the ambient occlusion from walls is too extensive?

Still present in 0.3.0:

Can replicate in 0.4.10, but Vanilla is a whole lot uglier actually. I'd call the reported behavior an improvement.

This is not solvable without completely reworking how lighting works. These artifacts are not a bug, they are a limitation of the current approach of per-vertex colors, and how colors get interpolated across surfaces. The only way to solve this would be to use a special tap filter and light map textures, rather than per-vertex colors. Doing so is not completely outside the realm of possibility for us (as it may have performance advantages), but right now it's not being considered.

For me on the latest commit the lines were faintly visible at 100% brightness.